The present invention relates to electrographic printers of the type wherein a printhead generates charge carriers and directs them at a recording or imaging member to form a desired image by the selective activation of electrodes. It is particularly directed to such printers wherein one set of electrodes is activated as a source of charge carriers, e.g., ions or electrons, and a second set of electrodes is activated to extract and accelerate the charge carriers toward the latent imaging member.
Printheads of this type are described in U.S. Pat. Nos. 4,160,257, 4,628,227, 4,992,807, and others. In the printheads described more particularly in the aforesaid patents, a set of electrodes are activated with an RF frequency signal of up to several thousand volts amplitude to create a localized corona or glow discharge region. Lesser control voltages are applied to one or more control electrodes located at or near the discharge region to gate positive or negative charge carriers from the region, and the printhead is biased with respect to a dielectric member to maintain an accelerating field therebetween, so that the charge carriers are drawn from the printhead and deposited as charge dots constituting a latent image on the dielectric imaging member as it moves past the printhead.
In printing devices using this type of printhead, the RF-driven corona generation lines extend along the width of the printhead, spanning many of the control electrodes, which cross them at an angle. One commercial embodiment, by way of example, has twenty parallel RF lines, which are crossed by one hundred twenty eight oblique control electrodes, known as finger electrodes. During the time when one RF line is activated by a burst of approximately five to ten cycles of a one to three MHz drive signal with a peak to peak amplitude of approximately 2700 volts, those finger electrodes which cross the RF line at the desired dot locations are activated to deposit charge dots.
In the conventional drive circuitry for such systems, the RF drive lines are actuated in a fixed sequence independent of the image being printed, while during any given RF line actuation, the number of finger electrodes which are actuated varies in accordance with the required number and location of dots for the pattern being printed. After a slight delay for the RF voltage to ramp up, the designated finger electrodes are turned on to cause charge carriers to pass from the printhead and accelerate toward the drum, belt or other latent imaging member. Specifically, during their "OFF" cycle, each finger is back biased by several hundred volts with respect to the screen voltage. During its "ON" cycle, the finger voltage is switched to approximately the same potential as the screen, so charge carriers of one polarity reaching the screen aperture are drawn to the imaging member.
In the original printers of this type, the finger electrodes were switched on for a fixed interval substantially co-extensive with the RF corona generation burst. Such operation produces a fixed amount of charge per actuation. More recently, in U.S. Pat. No. 4,841,313 or 60 Nathan K. Weiner, constructions with a finger pulse of varying duration have been proposed. This operation varies the amount of charge deposited at each dot. In U.S. Pat. No. 4,992,807 of Christopher W. Thomson, other control regimens involving varying voltage levels or potential differences in the front electrodes or electrode structures have been described.
Printheads of the aforesaid type are generally operated at a relatively small gap of about 0.25 mm from the image-receiving belt or drum surface, and are biased, with respect to the imaging member, to maintain a relatively high electrostatic acceleration field of 2-3 KV/mm in the gap. The size of the charged particle beams generated by the printhead decreases with higher acceleration field. Considerations of assuring a dependable firing threshold while not risking the occurrence of arcing generally prevent the use of extreme values for either printhead gap or acceleration field operating parameters, and dictate bias voltages and gap spacings in the range indicated above.
The operation of such closely-spaced imaging member and printhead electrode arrays at voltages effective to provide small beam dispersion requires voltages as high as fifty percent or more of the spark breakdown voltage, and may lead to erratic arcing or irregular toning of the latent image, so various attempts have been made in the past to reduce the potential difference across the printhead-to-drum gap. In U.S. Pat. No. 4,658,275 a construction is proposed that places a second screen electrode between the printhead and an imaging belt, with the second screen electrode and a conductor on the opposite side of the imaging belt maintained at the same potential to eliminate any electric field in the gap and prevent extraneous charging or toning of the belt. Others in the industry have proposed additional electrodes located closer to a drum with a grounded core, and maintained at potentials closer to ground to permit reduction of the printhead-to-drum gap, and hence limit the beam divergence.
Generally, charge-deposition printheads deposit a quantity of charge for each print dot in an amount that is sufficient to attract and hold toner onto the imaging member. For one typical dry-toned embodiment, the latent image surface potential required for toning may be between fifty and several hundred volts in charged areas. With such a significant potential, as charge is deposited on the imaging member, the latent image electric field builds up to such magnitude that the projected charge particle beam becomes increasingly divergent, so that the latent image charge dot spreads out. This charge spreading effect can result in the deflection of a substantial portion of the charge of one dot into an annulus outside of the intended dot area, "spreading" the dot dimension by several mils. When the printhead dimensions and operating parameters of a system have been optimized to print images with a resolution of several hundred dots per inch or more, the charge spreading effect can degrade print quality, resulting in loss of print density, loss of print detail, and blurring of color separation in multiply-toned prints.